Detail of Quantum Protocol BB84 : pat 1, Non son solo, siamo in Due
It is probably easier to learn it operatively with some excersize on an example.
* Alice's toolbox : Two sets of random number sequences. Spin generator
* Bob's toolbox : A set of random number sequence, Spin detector
* Eve's Toolbox : Arbitrary random number sequences, Spin generator, Spin detector
Here, we assume that the spin generator acts on two modes "0" and "2" of operator's selection, in each of which a bit "0"/"1" is translated in following manner into a spin state .
* "Mode 0" : "0" ~ | u >, "1" ~ | d >
* "Mode 1" : "0" ~ | r >, "1" ~ | l >
We also assume that spin detector has corresponding two modes for translating states back to bits.
Alice use the first random number sequence as the "origina" key sequence, which he translates into the spin state using the mode which is determined by the second random key sequence. Bob uses his random number sequence in deciding the mode for his reconstruction of key sequence from the back-translation of spin states.
* * * Alice * * *
| Orig Key |
0
|
0
|
1
|
1
|
1
|
0
|
1
|
0
|
1
|
0
|
1
|
0
|
1
|
| Mode |
0
|
1
|
0
|
0
|
0
|
1
|
0
|
1
|
1
|
0
|
1
|
1
|
0
|
| Crea St |
u
|
r
|
d
|
d
|
d
|
r
|
d
|
r
|
l
|
u
|
l
|
r
|
d
|
* * * Bob * * *
| Mode |
0
|
0
|
0
|
1
|
0
|
0
|
1
|
1
|
0
|
0
|
0
|
1
|
0
|
| Obs St |
u
|
d
|
d
|
l
|
d
|
d
|
r
|
r
|
d
|
u
|
u
|
r
|
d
|
| Recn Key |
0
|
1
|
1
|
1
|
1
|
1
|
0
|
0
|
1
|
0
|
0
|
0
|
1
|
When there is nobody in between Alice and Bob, if Alice and Bob look at those potion of the original key sequence and reconstructed key sequence for which encoding mode and decoding mode cooincide, they find perfect match between the original and reconstructed key sequences. One the otherhand, if they look at those potion where their modes differ, there will be 50% disagreement between original and reconstructed key.
Alice and Bob can now use shared key sequence to encode the message in classicVegenere protocol. |
Go To:
ResearchPage